Composite structures in aircraft typically utilize a tough surface skin layer supported by a lightweight core material. Development efforts to increase the strength/weight ratio of the core have resulted in cellular plastic structures such as rigid expanded foam of random cell pattern. Superior structural properties have been realized in cores formed in a geometric honeycomb pattern of hexagonal ducts, achieving very light weight due to the high percentage of air volume in the range of 90% to 98%. Such a core, when sandwiched between two skins, forms a structure possessing a uniform crushing strength under compression.
In known art, such cellular or ducted cores are commonly made from thermosetting resins. Utilization of such structures has expanded to include areas previously avoided due to structural demands and temperature, vibration and impact loading environments. Thermosetting resins, commonly used, have been found to lack the toughness and high temperature strength and stability needed for these applications.
New thermoplastic materials have offered improved properties; for example composite skin-surfaced structures having honeycomb cores made from preimpregnated thermoplastic fiber material provide excellent impact strength and damage tolerance. However, by their nature, the new thermoplastic materials require new and unconventional processing methods. As opposed to conventional thermosetting processes where sticky and viscous fluids are saturated into reinforcing fiber forms to be cured by catalysis and heat, thermoplastics, which have no cure cycle are hard and "boardy" initially, and have to be melted at high temperatures to be worked to the desired shapes. Thus completely different processing schemes are required for thermoplastics than those that have been developed for thermosets.
In known art, thermoset honeycomb material is made by a process that takes advantage of the flexibility of the reinforcing fabric before it is impregnated with resin. It is bonded and then expanded into hexagon honeycomb structure while it is soft, then wash coated with resin which is subsequently cured to give stiffness.
In contrast, thermoplastics, utilized in the present invention for their superior ultimate properties, are too viscous to be wash coated or by some other means saturated into the fabric after bonding the sheets together. The practical options for bonding thermoplastic core material together are further limited by the difficulty of making good adhesive bonds with thermoplastics. For these reasons, thermal fusion bonding of thermoplastic material into a ducted honeycomb structural pattern has been selected as the method for producing strong lightweight core materials in the present invention, which addresses new processing methods for realizing the full benefits of the superior ultimate properties of such structure.
Preimpregnated thermoplastic fiber material is available both in ribbon (continuous woven fabric sheet) and yarn form. The first step in processing preimpregnated thermoplastic fiber material into ducted honeycomb core blocks is to preform the thermoplastic material into a shape or pattern in preparation for bonding around hex mandrels into the desired ducted honeycomb pattern.
In a method of known art, thermoplastic ribbon is corrugated in a half hex pattern and cut into sheets which are stacked together with metal mandrels. The stack is pressurized and heated to fusion bond the material into a ducted matrix structure around the mandrels, which are then extracted individually in a press. Some disadvantages of this method are: (a) stacking is slow and erratic because the corrugated sheets tend to be springy and have to be coaxed into place, (b) individual mandrel extraction is slow, risky, and skill intensive, requiring a special long stroke thin pin pneumatic press, and (c) in the resulting structure, the wall thickness at the bonded interfacial facets is double that of the other walls, imposing 33% "dead weight" penalty because any strength from the double thickness walls is directional and does not contribute significantly to the overall useful strength.